An organic compound has various material systems compared with an inorganic compound, and has possibility to synthesize a material having various functions depending on the molecular design. Owing to these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
For example, a solar cell, a light emitting element, an organic transistor, and the like can be mentioned as examples of an electronic device using an organic compound as a functional organic material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light emitting element has been making remarkable development.
It is said that light emission mechanism of a light emitting element is as follows: when a voltage is applied between a pair of electrodes which interpose a light emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the light emitting layer, so as to form a molecular exciton and energy is released to emit light when the molecular exciton returns to a ground state. As excitation states, a singlet excitation state and a triplet excitation state are known, and light emission is considered to be possible through any of these excitation states.
Such a light emitting element has a lot of problems depending on a material in the case where an element property thereof is improved. In order to solve the problems, improvement of an element structure, development of a material, and the like are carried out.
As the most basic structure of a light emitting element, the following structure is known: a hole transporting layer formed of an organic compound having a hole transporting property and an electron transporting light emitting layer formed of an organic compound having an electron transporting property are stacked to form a thin film of approximately 100 nm thick in total, and this thin film is interposed between electrodes (see Non-Patent Document 1, for example).
A voltage is applied to the light emitting element described in Non-Patent Document 1, thereby light emission can be obtained from organic compounds having a light emitting property and an electron transporting property.
Further, in the light emitting element described in Non-Patent Document 1, functions are separately carried out. That is, a hole transporting layer transports a hole, whereas an electron transporting layer transports an electron and emits light. However, various interactions (for example, formation of exciplex, and the like) occur on an interface of stacked layers. As a result, a change in light emission spectrum or a decline in light emission efficiency may be caused.
In order to improve a change in light emission spectrum or a decline in light emission efficiency which is caused by the interaction at an interface, a light emitting element in which functions are further separately carried out is devised. For example, supposed is a light emitting element having a structure where a light emitting layer is sandwiched between a hole transporting layer and an electron transporting layer (see Non-Patent Document 2, for example).
In such a light emitting element as described in Non-Patent Document 2, a light emitting layer is preferably formed by using a bipolar organic compound which has an electron transporting property and a hole transporting property so that interaction caused at an interface is further suppressed.
However, most organic compounds are monopolar materials having either a hole transporting property or an electron transporting property.
Therefore, a bipolar organic compound having both an electron transporting property and a hole transporting property has been required to be developed.
In Patent Document 1, a bipolar quinoxaline derivative is described. However, since characteristics such as heat resistance are not sufficiently obtained yet, more various bipolar organic compounds have been required to be developed.
[Non-Patent Document 1]
    C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)[Non-Patent Document 2]    Chihaya Adachi et al., Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)[Patent Document 1]    PCT International Publication No. 2004/094389